Ground fault detectors are used in high voltage circuits such as in ballasts which drive gas discharge tubes such as neon display tubes. A ground fault detector is used to safeguard current carrying circuits, detecting leakage current to ground and shutting off the power supply of the ballast in the event the leakage current exceeds a predetermined value. Excessive leakage current can cause arcing, which can cause fire or can be lethal, and therefore maximum leakage is regulated by standards.
One of the conventional ways of controlling the shut-off of the high voltage is to monitor the alternating current which is returned to ground carried by the center tap of the high voltage secondary winding of a high voltage transformer. In the event the high voltage leads of the secondary winding are conducting to ground, causing current to flow from the center tap to ground, a current transformer reflects this back to a shutdown circuit. Examples of circuits which use this principle are described in U.S. Pat. No. 4,663,571 to Nilssen and U.S. Pat. Nos. 4,613,934 and 5,089,572 to Pacholok.
Another conventional way of controlling shut-down of a high voltage circuit is to detect the inherent phase shift between current and voltage when the high voltage is radiated capacitively to ground. However, the realized circuit requires a phase discriminator circuit and a high parts count, which is costly.
A typical ground fault detector is comprised of a solid state switch which accepts a trigger voltage and conducts to operate a relay, etc. when the trigger voltage is exceeded. The relay operates switch contacts in the power supply, shutting down the power supply. The trigger voltage is derived by detecting the leakage current and converting this current to a voltage which is rectified and is applied to the solid state switch, which will operate if the voltage, and thus the current, is large enough. Ground fault detector circuits of this type are described, for example, in U.S. Pat. No. 4,114,089 to Ahmed, U.S. Pat. No. 3,899,717 to Legatti et al and U.S. Pat. No. 4,138,707 to Gross.
The leakage current detected in the aforenoted structures constitutes radiated or reactive alternating current, similar to current emitted from a radio frequency transmitter. The return energy is purely capacitive to ground. If the energy emitted by both high voltage leads (e.g. the antennae) of the secondary winding of the high voltage transformer of the ballast is not balanced capacitively, a current will flow through the center tap of the secondary winding to ground, creating an A.C. voltage which is detected as leakage current, and causing a false shutdown of the power supply.
It has been determined that hazardous arcing to ground can be detected solely from the D.C. current flowing from a D.C. biased winding to ground, rather than from the A.C. reactive current to ground. The systems described above shut down in the presence of A.C. reactive current, even without the presence of additional resistive current, which causes the false shutdown. The prior art systems are thus not reliable detectors of the hazardous currents.